Hi Louisa, Ive never heard of anyone being asked to do the cambridge diet, or any other VLCD prior to their op. Some bariatric teams though do insist on some weight loss before going ahead with the op, and reading the replies on this thread it would seem that L&D are one of those teams.
I would expect though that you would have to do whatever diet you wanted to (ww, sw, calorie counting or whatever) to achieve the 5% (if indeed that is the target set) by whatever date they ask you to lose the weight by.
As a seperate issue, it is then normal to have to go on a pre op diet which isnt about weight loss, but about shrinking the liver. This is normally for 2 weeks prior to the op.
Hope that clarifies things a little.